In order to be compared, your tests must be performed under similar conditions. Therefore, the app is constantly checking:
The ambient light. Try to do your tests in a well-lit room, with no direct light on your face or on your device. If the room is too dark or too bright, the app will tell you to find another location to do the test.
The distance to screen. During the tests, the distance between your device and your face must be between 14 to 18 inches. This distance is checked before and during the tests.
If you struggle to keep your device at the right distance, make sure it is on a stable surface, such as on the blue OdySight box you received when your physician prescribed the app. If you are having difficulties getting the app to detect the correct distance or find your face, make sure you are covering the eye you are not testing with your hand held horizontally, and not vertically.